1. Field of the Invention
The present invention relates to an ignition timing controlling device for an engine, which controls an ignition timing of an engine based on a basic ignition timing map.
2. Discussion of Background
Conventionally, an ignition timing controlling device for controlling an ignition timing of an engine, employs a map (a basic ignition timing map) wherein an optimum ignition timing is recorded, in controlling the ignition timing to a target ignition timing.
In this basic ignition timing map, the ignition timing which is optimal to an engine, is classified by a charging efficiency and a revolution number of an engine. Further, the conventional ignition timing controlling device detects the charging efficiency and the revolution number of an engine as running state parameters of an engine. The ignition timing data which correspond to the running state parameters, are read from the basic ignition timing map, based on which the ignition timing is controlled.
The two-dimensional basic ignition timing map which is employed in the conventional ignition timing controlling device, is constructed as shown in FIG. 5. In the basic ignition timing map shown in FIG. 5, values proximate to an optimum ignition timing are set when an engine is in a completely warm-up state, generally, data which are more advanced are set when the revolution number of an engine is high and the charging efficiency is low, and conversely, data which are more retarded are set when the revolution number of an engine is low and the charging efficiency is high. Especially, values which are considerably retarded compared with the optimum ignition timing, are set to prevent knocking, since the knocking is apt to cause in case wherein the revolution number of an engine is low and the charging efficiency is high.
In the above conventional ignition timing controlling device, when the cold-starting is performed in case wherein the atmospheric temperature is extremely low (for instance, -20.degree. C.), the viscosity of an engine oil is extremely large at the extremely low temperature, and therefore, the revolution number of an engine is not elevated at once and is gradually elevated. Further, the charging efficiency is provided with a high value since the intake quantity increases at a low temperature due to the fast idling. Accordingly, ignition data which are comparatively retarded in a region of a low revolution number of an engine and a high charging efficiency, are read, as the ignition timings which are read from the basic ignition timing map. Further, the ignition timing is gradually advanced as shown in a broken-line arrow mark "a" in FIG. 5. In FIG. 5, the broken-line arrow mark "a" designates a locus wherein the charging efficiency E.sub.c and the revolution number N.sub.e (actual revolution number) of an engine change in the extremely low temperature starting, whereas a solid-line arrow mark "b", a locus wherein the charging efficiency E.sub.c and the revolution number N.sub.e (actual revolution number) of an engine change in the ordinary temperature starting.
However, generally, the combustion rate is retarded when the atmospheric temperature is low and the knocking is hard to cause. Therefore, the optimum ignition timing is to be provided with an advanced value compared with that which is set in the basic ignition timing map, and therefore, the ignition timing is considerably retarded compared with the optimum ignition timing in performing the cold-starting at an extremely low atmospheric temperature when the basic ignition timing map is employed.
When the ignition timing is retarded compared with the optimum ignition timing in performing the cold-starting, the elevation rate of the revolution number of an engine is low as shown in a curve A in FIG. 6(b), and the engine is apt to stall. FIGS. 6(a) and 6(b) are graphs showing a state of an engine in starting up the engine. FIG. 6(a) designates a change of the ignition timing in starting up an engine, and FIG. 6(b), a change of the revolution number of an engine in starting up the engine.